An important aspect in the manufacture of paper products is the whiteness of the paper. Traditionally, one means of increasing the whiteness of paper has been through the use of fluorescent whitening agents (FWAs). FWAs absorb light in the near ultraviolet (UV) and emit this light in the visible spectrum, thereby increasing the amount of visible light reflected from the paper, and thus increasing the apparent whiteness of the paper.
The presence of FWAs in manufactured paper, however, is limited by federal regulations, particularly in paper products destined to contact food products. Federal Regulation 21 CFR .sctn.176.260 prohibits the presence of any harmful or deleterious materials that can migrate to food products from recycled papers used for food packaging.
Recycled paper is frequently used in the manufacture of new paper products, which poses several potential problems. First, recycled paper often contains FWAs, which, as discussed above, are forbidden in products destined for food-contact. Second, recycled paper arises from many different sources and the whiteness of the final manufactured product will vary dramatically depending on the amount of FWAs present in the starting materials. Third, for optimal performance of a paper mill, processing recycled paper requires the amount of fluorescence in the pulp going to the mill to be constant. Therefore, the recycling of paper products which have been previously treated with FWAs cannot be undertaken without an effective method for destroying the FWAs (Dubreuil, Progress in Paper Recycling, August, 1995, 98-108).
The principal FWAs used in the paper industry are diaminostilbenes which have the general structure represented in Formula I: ##STR1##
One common FWA is Tinopal SFP, which has the structure of Formula I wherein R.sub.1 is NH.sub.2 and R.sub.2 is NH(CH.sub.2 CH.sub.2 OH). Other potential R.sub.1 substituents include, but are not limited to, N(CH.sub.3)(CH.sub.2 CH.sub.2 SO.sub.3 H), and OCH.sub.3. Other potential R.sub.2 substituents include, but are not limited to, N(CH.sub.2 CH.sub.2 OH).sub.2, NH(C.sub.6 H.sub.4 SO.sub.3 H), and N(CH.sub.3)(CH.sub.2 CH.sub.2 SO.sub.3 H). Other FWAs are known and have the general structure as shown in Formula II, below. ##STR2##
Several techniques are known for the removal of FWAs from recycled paper. For example, oxidizing agents, such as chlorine, destroy the stilbene dyes which make up most FWAs, rendering them nonfluorescent. The destruction of FWAs can be assessed by monitoring the fluorescence of a sample. As the FWAs are destroyed, the sample's fluorescence decreases. Increasingly stringent environmental regulation of organic halides imposes limits on such use of chlorine in the removal of FWAs from paper-containing samples.
Other non-chlorine reagents have been evaluated for their potential use in the destruction of FWAs, including sodium hydrosulfite, formadine sulfinic acid, and hydrogen peroxide; however, each of these had the reverse effect of actually increasing the measured fluorescence of the samples. Ozone has also been used as an agent to destroy FWAs. Although ozone does reduce the fluorescence of test samples, unacceptably high levels of ozone are required to reduce fluorescence. Ozone also has the undesirable effect of degrading cellulose, resulting in a paper product with decreased strength properties. These disadvantages, coupled with the high cost of ozone, make the use of ozone impracticable.
There is, therefore, a need for methods of reducing fluorescence in paper-containing samples that do not suffer from the drawbacks of existing methods. There is a further need for methods of reducing fluorescence in paper-containing samples that contain FWAs. There is also a need for improved methods of destroying FWAs in paper-containing samples.